Spark plugs have been around as long as internal combustion engines have, and are often a misunderstood component. This basic guide is designed to assist the technician, hobbyist, or race mechanic in understanding, using, and troubleshooting spark plugs.

Spark plugs are a "window" into the engine, and can be a valuable diagnostic tool. The spark plug displays the condition inside the combustion chambers of the engine. The experienced tuner can use spark plugs to find the root cause of problems, determine air-fuel ratios, and to increase vehicle performance.

Spark Plug BasicsThe primary function of the spark plug is to ignite the air/fuel mixture within the combustion chamber under any operating condition.

Spark plugs must provide a path and a location for electrical energy from the ignition coil to create a spark used to ignite the air-fuel mixture. A sufficient amount of voltage must be supplied by the ignition system to spark across the spark plug gap. This is called “Electrical Performance.”

The spark plug firing end temperature must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called “Thermal Performance”, and is determined by the heat range selected.